I see that quercia is the singular form of oak. What is the plural form? Is it querce? Is it querche?

First you find weather you have a masculine or femminine noun. Nouns ending in -o are masculine, nouns ending in -a are feminine, so you have a feminine noun. Then you have to know how to congugate them. So for example: il biglietto becomes I biglietti (the ticket, the tickets) la strada becomes le strade (the road, the roads) -ca becomes -che, -ga becomes -ghe, -co becomes -chi, -go becomes -ghi, -io becomes -i, -cia becomes -ce, -gia becomes -ge.
Look for the right ending, know what it becomes when plural and there you go.

Quercie